Various embodiments relate to dimmable electronic ballasts, that is to say electronic ballasts that can be controlled in such a way that the total power provided by them can be varied in a wide range. For example, an electronic ballast that is configured for a maximum power to be provided of 300 W provides a total power of 180 W at 60%, and of approximately 60 W at 20%.
FIG. 4 carries the variation in the switching frequency fPFC (PFC=power factor correction) of, for example, three parallel connected PFC stages of a generic electronic ballast as a function of the partial power to be provided by each PFC stage. Thus, given a provided total power PGes of 300 W, that is to say each PFC stage provides 100 W, the switching frequency fPFC is 74.6 kHz. Given a total power to be provided of 240 W, that is to say each PFC stage provides a partial power of 80 W, the switching frequency fPFC is 93.2 kHz. Given a total power to be provided of 120 W, that is to say each PFC stage provides a partial power of 40 W, the switching frequency fPFC already rises to a considerable 186 kHz. Finally, the switching frequency fPFC even increases to 372.9 kHz given a total power to be provided of 60 W, that is to say each PFC stage provides 20 W.
The following, further parameters that are reproduced in FIG. 4 relate to the following variables: Unetz is the rms value of the alternating supply voltage connected between the two input connections. Inetz is the rms value of the current flowing via the input connections. Inetzmax is the maximum value of the current flowing via the input connections Urail is the DC operating voltage provided at the output of the rectifier. The middle block of FIG. 4 reproduces variables that relate to the PFC stages, that is to say the partial devices for the power factor correction. Thus, L is the quantity of the inductor used in a PFC stage. IL,max is the maximum current flowing through this inductor. Ton is the switch on time of the switch of a PFC stage, Toff is the switch off time of the switch of a PFC stage. Finally, the last column of FIG. 4 specifies how many PFC stages are simultaneously in operation in order to attain the respective provided total power.
In order to be able to satisfy the EMC standard decisive for electronic ballasts, presently EN55015, it is required to design the EMC filter such that it is effective for the entire power range, that is to say for a very wide range of the switching frequency fPFC. There is a consequential increase in the complexity and costs of the EMC filter. Moreover, the switching frequency fPFC increases considerably for very small total powers that are to be provided, such that in this case the efficiency of the power factor correction device decreases markedly in an undesired way.